Process for preparing a layer of compounds of groups ii and vi

ABSTRACT

A process for preparing a layer of compounds of Groups II and VI characterized in that a layer of the compounds is formed on the surface of a substrate, and the formed layer is heat-treated in an atmosphere comprising an inert gas, 0.1 - 10 percent by volume of oxygen on the basis of said inert gas and vapour of at least one element selected from a group consisting of sulfur, selenium and tellurium so that uniform grain growth with a narrow range grain size distribution is effected.

United States Patent 1191 Shimizu et al.

1451 Feb. 19, 1974 PROCESS FOR PREPARING A LAYER OF COMPOUNDS OF GROUPS II AND VI {75] Inventors: Kazuo Shimizu; Okio Yoshida, both of Yokohama; Kazuo Terakawa, Yokosuka; Satoshi Aihara, Yokohama, 'all of Japan [73] Assignee: Tokyo Shibaura Electric Co., Ltd.,

Kawasaki-shi, Japan [22] Filed: Aug. 17, 1971 [21] Appl. No.: 172,454

[30] Foreign Application Priority Data Aug. 17, 1970 Japan... 45-71391 52 US. (:1 117/201, 117/62, 117/106 A 51 Int. Cl B44d 1/02 [58] Field of Search 117/106 R, 62, 215, 106 A,

[56] References Cited UNITED STATES PATENTS 3,622,381 11/1971 Ohnishi et al 117/106 R OTHER PUBLICATIONS Kitamura, S. Influences of Heat-Treatment Upon Cadmium Sulfide Single Crystals, in J. of Phys. Soc. of Japan 16(12): p. 2,430-2,439, Dec. 1961.

Primary Examiner--Cameron K. Weiffenbach Attorney, Agent, or Firm-Kemon, Palmer & Estabrook [57] ABSTRACT 4 Claims, 3 Drawing Figures PATENIEI] FEB I 91974 sum 2 BF 2 FIG. 2

iii

PROCESS FOR PREPARING A LAYER OF COMPOUNDS OF GROUPS I] AND VI FIELD OF THE INVENTION This invention relates to a process for preparing a layer of Groups II Vl compounds with narrow grain size distribution.

DESCRIPTION OF THE PRIOR ART A film of a large area of a compound or a film of a compound separately distributed over a large area is useful as a light-electricity transducer in a wide range of technical fields, such as solar cells, solid state photosensors for image pickup application, photoconductive film for image pickup tubes, etc. Such films should be furnished not only with electrical properties satisfactory enough for desired performance, but also with uniformity in such properties over a wide area. For instance, local unbalance in photo-sensitivity or intensity of luminescence of a film element not only abates the commercial value thereof, but constitutes a fatal factor which restricts the life of the film element.

Usually such a film is formed by vacuum evaporation or gaseous phase reaction on the surface of a substrate plate. In order to improve the electrical properties thereof, however, heat treatment is necessary after film formation in many cases. Such heat treatment is generally accompanied by growth of grains which constitute the film. Therefore the characteristics of a film element, especially uniformity of the characteristics over a wide area is often influenced by the grain growth during the heat treatment. In order to secure desirable grain growth in the heat treatment, it is important to select a suitable temperature, but the maximum temperature may be limited because of heat resistance of the substrate material, possible decomposition of the compound constituting the film, etc. Therefore, fluxes are widely employed in such. heat treatment.

A typical example of such fluxes used in the heat treatment is cadmium chloride, which is employed in the treatment of cadmium sulfide used for photoconductive elements. Cadmium chloride is effective for cadmium selenide or cadmium sulfoselenide film, too. To make the effect of the heat treatment satisfactory, consideration is paid to making the flux concentration as uniform as possible. However, when cadmium chloride is used in the heat treatment of cadmium sulfide or cadmium selenide photoconductive films, a large quantity of chlorine penetrates into the photoconductive film and in some cases the concentration thereof reaches as high as 10 atoms/cc. This chlorine builds up a shallow doner level in the cadmium sulfide or selenide. The existence of such high concentration of a shallow doner level impairs photo-sensitivity of the films and in some other cases retards response to light.

The advantage of the above-mentioned heat treatment in which the flux is used is that grain size is rather uniform after grain growth. In contrast, in methods in which flux is not used, but, for instance, simply temperature is raised for promotion of grain growth, the grain growth rate varies from part to part and a film with a wide range distribution of grain size results. This is the cause of non-uniformityin film thickness and causes fatal defects such as pinhole, peeling-off of film, etc.

In order to carry out the uniform grain growth of a film which comprises amorphous or extremely fine grains, it is necessary in general to generate a suitable number of uniformly distributed nuclei on the surface of the film.

SUMMARY OF THE INVENTION In accordance with this invention, such uniformly distributed nuclei which lead to a film of a uniform structure with narrow range grain size distribution are provided on the surface ofa film of Groups II VI compounds by carrying out the heat treatment of said film in an atmosphere comprising an-inert gas and 0.1 10 percent by volume of oxygen on the basis of said inert gas and the vapour of at least one element selected from a group consisting of sulfur, selenium and tellurium.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF PREFERRED EMBODIMENTS Now the invention is explained in detail with respect to an embodiment pertaining to a preparation of a photo-conductive element. A thin film of cadmium selenide, 5p. thick for instance, is deposited on the surface of a substrate plate kept at I50" C in vacuo by vacuum evaporation technique. The thus obtained film is heattreated in an inert gas such as nitrogen containing 5 percent by volume of oxygen at 500C for 1 hour,

wherein selenium vapour exhibiting saturation vapour pressure at 500C co-exists. This selenium vapour can be mixed with the oxygen prior to the heat treatment or can be generated in situ by suitable means. By this heat treatment, the grain size of the originally deposited film which is about 1,000 A grows to about 1.5g. and a film having grain size which is quite uniform is obtained. This grain growth is the same as the grain growth which results from a process in which 20 mole percent of cadmium chloride is mixed with cadmium selenide when the substrate plate is treated therewith and the deposited film is heat-treated in a nitrogen atmosphere.

The effect and advantage of this invention is now given below on the basis of the above-described em bodiment. The point of this invention is that limited volume of oxygen is employed in combination with selenium vapour in the heat treatment atmosphere. To

heat-treat cadmium selenide in a selenium vapour has been known and practiced in the prior art. However, this is a treatment for the purpose of filling the selenium vacancies in cadmium selenide film, and is car ried out so as to modify the electrical properties of a film. Also heat treatment in air is frequently employed as the treatment in an oxygen=contuining atmosphere. Air is regarded as a mixture of oxygen and nitrogen. Japanese Pat. Publication No. 23456/65 refers to nitrogen atmosphere containing 0.2 1.7 percent oxygen as the atmosphere containing limited amount of oxygen.

3 However, the invention of this reference relates to a process of heat treatment in which cadmium selenide is heat-treated together with a flux, and the atmosphere comprises oxygen, nitrogen and cadmium halide vapour, whereby grain growth is effectuated with the cadmium halide flux, and oxygen is added for filling selenium vacancies merely as anauxiliary agent.

In accordance with this invention, as illustrated in the above example, oxygen and selenium vapour must co-exist in the atmosphere-during the heat treatment. That is, if the atmosphere lacks either of the two, no such film as is aimed at in this invention can be obtained. For instance, if the operation of the abovementioned working example is carried out in an atmosphere which contains no selenium vapour, grain growth does not proceed as shown by comparison of Curve 1 in FIG. 1, which represents the grain size distribution before the heat treatment, and Curve 2, which represents the grain size distribution after the heat treatment. On the other hand, when oxygen is omitted from the atmosphere, the grain size distribution after the treatment is represented by Curve 3 in FIG. 1. In this case, the range of grain size distribution is much enlarged.

In contrast, Curve 4 represents the grain size distribution when a film is heat-treated in an atmosphere in which oxygen and selenium vapour co-exist in accordance with this invention. Comparison of FIGS. 2 and 3, which show electron photomicrographs of the film of this invention and that of the prior art, shows how superior the film of this invention is to that of the prior art in the grain size and the grain size distribution. Thus, the photoconductive film of this invention, as represented by FIG. 2, is free from unevenness in photoelectric sensitivity.

Now the reason why such effect is achieved by this invention is discussed with particular reference to the abovementioned embodiment. In the course of the heat treatment, when the temperature of the substrate plate is raised, oxygen and selenium from the atmosphere and cadmium from the film react on the surface of the film to form an intermediate oxide, for instance, cadmium selenite (CdSeO which is a salt of an oxyacid. This compound is redecomposed at higher temperatures, and therefore, transient formation and decomposition of the intermediate oxide takes place in the course of temperature raise. This gives nuclei which become sites where grain growth is initiated, and rearrangement of structure begins at the surface and it proceeds into the inner parts. Such intermediate steps promote grain growth and bring about narrow distribution of grain size. I

If the atmosphere in which'there is to be conducted the desired reaction should contain larger proportions of oxygen than prescribed for concomitance with vapours of selenium, for example, if the reaction is performed in the ordinary air, then oxidation will proceed in excess. This will cause an intermediate oxide to form whose growth should originally stop with the formation of a nucleus prominently to settle on the treated layer to constitute an objectionable surface deposit, with the resultant deterioration of the electric properties of said layer. This is the reason why the amount of oxygen in the heat treatment atmosphere must be limited when the synergistic effect of oxygen and selenium is expected. This limit is 10 percent by volume on the basis of the volume of base or carrier gas such as argon or nitrogen. It is needless to say that too small an amount of oxygen does not work. In order to effectuate smooth creation of grain growth nucleus sites, at least 0.1 percent by volume of oxygen is necessary.

On the other hand it is rather difficult to determine a suitable amount or concentration of selenium vapour, since the partial pressure of selenium vapour in the heat treatment atmosphere is related to the electrical properties of finished products, but at least I mm Hg will suffice. A temperature of at: least 450C for the heat treatment is required since the treatment is carried out without the aid of a flux. I

The invention has been explained above with respect to cadmium selenide layer in particular. However, this invention is effectively applicable to compounds of Groups II and VI in general such as cadmium sulfide, cadmium telluride, zinc sulfide, zinc selenide, zinc telluride, and a mixture (solid solution or heterogeneous mixture), and films of multi-layer structure of these compounds, too. Layers of these compounds may contain known impurities. The inert gases which constitute the main component of the heat treatment atmosphere are argon, nitrogen or a mixture thereof. In conclusion, the process of this invention is characterized in that nuclei necessary for uniform grain growth are provided on the surface of a film to be treatedby causing formation and decomposition of intermediate oxide, such as salt of an oxyacid, on the surface of the film in thecourse of the heat treatment.

According to this invention,'grain growth is effected without using any flux, the film to be treated isnot contaminated with the flux, and therefore, control of impurity content is much simplified.

What we claim is:

1. A process for preparing a layer of Group II-VI compound selected from the group consisting of CdX and ZnX, wherein X is an element selected from the group consisting of selenium, sulfur and tellurium which comprises forming a layer of said compound on a substrate plate and heat-treating the resulting layer in an atmosphere comprising an inert gas, 0. ll0 percent by volume of oxygen on the basis of said inert gas and vapor of said group element X whereby uniform grain growth with narrow range grain size distribution is effected. I

2. The process of claim 1 wherein said inert gas is argon or nitrogen.

3. A process for preparing a layer of compounds of Groups II VI characterizedin that a CdSe layer is formed on a substrate plate, the thus formed CdSe layer is heat-treated in an atmosphere comprising an inert gas, 0.1 10 percent by volume of oxygen on the basis of said inert gas and the vapour of selenium at a temperature not lower than 450C, whereby uniform grain growth with narrow range grain size distribution is effected.

pressure of the selenium vapour is not less than 1 mm Hg at the heat treatment temperature. 

2. The process of claim 1 wherein said inert gas is argon or nitrogen.
 3. A process for preparing a layer of compounds of Groups II -VI characterized in that a CdSe layer is formed on a substrate plate, the thus formed CdSe layer is heat-treated in an atmosphere comprising an inert gas, 0.1 - 10 percent by volume of oxygen on the basis of said inert gas and the vapour of selenium at a temperature not lower than 450*C, whereby uniform grain growth with narrow range grain size distribution is effected.
 4. A process as set forth in claim 3 wherein the partial pressure of the selenium vapour is not less than 1 mm Hg at the heat treatment temperature. 